Modern technology demands the fabrication of ever smaller and more precise structures for electronic circuits, optical components, micro electromechanical structures (MEMS), and other applications. Many such structures and devices, on the scale of micrometers or nanometers, are fabricated on silicon wafers using photolithographic methods.
A typical photolithography technique involves depositing a thin layer of a photosensitive material called a “photoresist” onto the surface of a semiconductor substrate such as a silicon wafer, by a process called “casting.” Photolithographic imaging is then used to transfer a desired pattern, designed on a photolithographic mask, to the photoresist by selective exposure to a radiation source such as light. The photoresist is then chemically developed to remove the radiation exposed areas (in a positive resist) or the unexposed areas (in a negative resist), leaving behind a pattern of photoresist to protect specific parts of the substrate during subsequent processes such as etching (removing material), deposition (adding material to the substrate surface), or diffusion (diffusing atoms into the substrate). Etching can be performed, for example, using a reactive chemical, sometimes in the form of a plasma. A plasma can also be used to sputter material from a surface by causing charged particles from the plasma to impact the surface with sufficient momentum to displace surface molecules. Deposition can be performed, for example, by chemical or physical vapor deposition or plasma enhanced chemical vapor deposition. After processing, the patterned photoresist is removed. Lithography process are time consuming and, while efficient for processing a complete wafer, are less useful for localized processing.
Focused beams, such as focused ion beams (FIBs), electron beams, and laser beams, are also used for forming small structures. While being able to form extremely precise structures, processing by such beams is typically too slow to be used for mass production of fine structures. FIBs can be used to sputter a substrate surface because they employ a relatively large ion such as, for example, a gallium ion (Ga+) that can be accelerated easily to achieve the momentum needed to displace molecules of the substrate. FIBs can also be used with a precursor gas to enhance etching chemically or to deposit a material onto the surface. Electron beams can also be used, together with an assisting precursor gas, to give rise to etching or deposition processes.
Electron beam, FIB, reactive gasses and plasma processes can be used either alone or in combination with one another to manipulate substrate surfaces, for example, to create and repair photolithographic masks. Reactive gasses typically also exhibit material selectivity. These processes can provide varying degrees of fabrication tolerances, material characteristics, processing times and machining flexibility.
Many problems still exist, however, with the methods of fabrication as currently used and described above. For example, it is difficult to precisely fabricate high aspect ratio holes, that is, holes having a depth that is much greater than their widths. Because current fabrication processes cause holes or trenches to become wider as they are etched deeper, adjacent deep features must be spaced further apart than desired. Time consuming photolithography processes are efficient for processing entire wafers, but are not useful for processing local regions on a wafer. Conversely, direct-write FIB and electron beam induced processes are efficient for highly localized processing employed in nano-prototyping, circuit edit, and photolithographic mask repair, but are not useful for processing entire wafers.